Coupling circuit for helical delay lines



July 16, 1957 R. w. PETER COUPLING CIRCUIT FOR HELICAL DELAY LINES Filed Aug. 2e, 1952 2 Sheets-Sheet 1 /TTOR NE Y July 16, 1957 R. w. PETER 2,799,797

coUPLING CIRCUIT FOR HELICALDELAY LINES Filed Aug. 29, 1952 z'sheets-sheet 2 fl 'I'TOR Y 2,799,797 Fate-rated Juiy 16, 1957 COUPLNtG CllRCUiT EUR HELICAL BELAY ELENES Reif W. Peter, Princeton, N. 5., assigner to .Radio Sorporation oi America, a corporation of Belair/are Application August 29, 1952, Seriai No. 397,950

Claims. (Ci. S15-3.5)

The present invention is related to `a coupling circuit for a helix transmission line, and particularly to a coupling circuit especially well adapted for coupling to `the helix of a travelling wave tube.

Various coupling circuits have been proposed heretoytore for coupling to the helix of a travelling Wave tube. These couplings have been ecient, but not as eihcient as -desired where a 'coaxial line is to carry the `signal into or out of the helix. Where eicient couplings are known Ithat might be employed, they detract from the operation of the tube by their undue length 'between the electron gun input end (or collector on the output end), :and the interaction space `along the helix.

In my copending application Serial No. 291,225, tiled June 2, 1952, now Patent No. 2,758,243, land in the copending application of Wellesley I. Dodds, Serial No. 291,189, now Patent No. 2,758,244, are `disclosed and claimed means for coupling to a travelling Wave tube helix which in considerable measure alleviate the `abc-vement-ioned diculties. However, these means, as disclosed in the said applications, :are not especially adapted -for coupling from coaxial lines but are disclosed for hollow pipe wave guide couplings to a helix.

`It is an object of the present invention to preserve the advances taught in the above-mentioned applications, and to improve thereon by adapting the teachings of the said applications to coupling a coaxial line `to a helix in an advantageous and desirable manner.

`It is another object of the invention to provide ,an improved coaxial line-to-helix coupling.

A further object of the invention is to provide such a coaxial line to helix coupling which is improved in efciency over, and provides better matching qualities than, prior coaxial rline to helix couplings.

vIt is still another object of the invention -to provide a coaxial line to helix coupling which has improved qualities of compactness to allow close positioning of an elect-ron gun when the helix is employed in a travelling wave tube.

According to the invention, a resonant section Iof `coaxial line has an inner conductor-gap- A helix is coupled to the resonant section by a coupling probe connected to the helix at lan end thereof and either extended ibeyond this end of the helix or folded back, and coacting with the fields in the resonant Asection at the inner conductor gap, The inner conductor ofthe section is tubular to permit passage of an electron beam. Preferably, the -arrangement permits positioning of the helix and the electron gun of `a travelling Wave tube 'close to each other. Preferably, the helix probe is conductively connected to a capacitive sleeve which couples capacitively to the inner conductor or an end wall of the resonant section by ybeing positioned coaxially of the resonant line section but spaced from any of the resonant boundary walls, either that of the inner or that of the outer conductor.

The foregoing objects and other objects, advantages, and novel features of the invention will be more apparent from the following description when read in connection with the accompanying drawing, in which like reference numerals refer to like parts, and in which:

ig. l is a longitudinal cross-sectional view of a complete travelling wave tube illustrating one :preferred lembodiment of the invention as constructed, with the like couplings at input and output, each coupling having a folded-back coupling probe and a capacitive coupling sleeve, the sleeve being 'capacitively coupled to the inner conductor and internally there-of;

Fig. 2 is `a longitudinal cross-sectional view of another coupling Varrangement according to the invention using an extended probe connected to the helix and a capacitive `sleeve similar to that of Fig. l, and showing a portion of a travelling wave tube with helix coupled thereto;

Fig. 3 is a longitudinal cross-sectional view of still another `embodiment of the invention, in which an extended iccupling probe `but no `capacit-ive sleeve is employed;

Fig. 4 is a longitudinal cross-sectional view of :another `embodiment oi the invention using a folded-back probe and omitting a wall of the coaxial resonator which is unnecessary for its operation, for reasons rdiscussed hereinafter more lfully;

Fig. 5 `comprising as parts Figs. 5a to 5]' inclusive shows, in Figs. 5a and 5j, in longitudinal cross-sectional views, alternative coaxial resonator portions with each of which may be combined any one of the forms of helix couplings `with probes shown in Figs. 5b to 5j inclusive.

Referring to `Fig. l, a travelling wave tube amplifier has an input coaxial line lil with inner `conductor 12 and outer 'conductor 14. A cylindrical shield 16 is -surrounded by a magnet winding 1S. in an envelope 2t) is mounted at one bulbous end an electron gun 22. The envelope 2t? has a suitable ibase 24 with pins for applying voltages to the gun elements (not separately shown). A spring 26 mounted on shield 16 presses the enevlope against `a bracket Z3 mounted 4on theV shield yopposite `spring 26 to protect and hold tirmly the envelope '20.

An annular stop 3d tightly titted linto the shield 16 positions the shoulders of the lbulbous end 4of the envelope 2i?. A metallic spiral shield 32 (although helical in form, not to 'be confused with the travelling Wave helix) is 'connected at one end `to the iinal `accelerating electrode 23, of the electron gun 22 and at the other `end to a capacitive `sleeve 34.

A resonant section or resonator 36 of coaxial line has `as one portion a cup `shaped member 38 having a central opening and as a second portion a second member 46 which may rbe described as a section of `coaxial line shortcircuited at `one end, open-circuited at the 4other end 40a, and with a tubular center conductor. The resonator 36 is completed by tting the -two members 3S and 4@ into the shield 16 with the `cup concavity of member 3S and the open -circuited end of second member 46 facing `and next adjacent each other. The resonator 36 wall between the Iouter conductor portion of second member d@ and the cup Walls of cup member 33 is vcompleted -by the cylindrical `shield 16 with which both members 38 and 4i) make contact. Also the opening in the cup shaped member 33 and the tubular center conductor and opening thereof of second member 40 are heldin coaxial alignment.

Energy may be supplied to this resonator 36 by the coaxial line 10 the outer `conductor 14 of which is in contact with shield 16 from the tube input-output end (right end as viewed in the drawing) through a side opening at the periphery of the second member `40 into the resonator 36. At :about the longitudinal mid-point (though other points may be selecte-d) in the resonator cavity, the outer conductor 14 is terminated. The inner conductor 12 is extended in a right angle fturn to form a coupling loop 12a which at its end contacts near the is a gap 46 between the inner conductor comprising the Vend 40a of second member 40 inner yconductor and the cup shaped member 38. The spiral shield 32 extends in Y contact With the envelope along the opening length across this gap 46. The contact with the end of sleeve 34 is Ymadepreferably at the same point as the end of the opencircuited end 40a of the second member 40 center conductor. The sleeve 34 is held by the envelope, which supports it in capacitive relationship to the inside of the Ycenter conductor of coaxial member 40. A coupling probe V42 is folded back from the end 44a of a travelling wave helix 44. The folded back coupling extends across the gap and may,'if desired, be in contact with the spiral Ashield 32 along the gap 46. The wave helix44 may start at its connection toprobe 42,1and in fact may be a continuation of the wire of which the probe is made. The wave helix proper may begin in the plane of the base of cup member 38. It is then continued toward the output end through a cylindrical tubing 43 preferably of quartz, which functions primarily as a support. At the input end the tubing 48 is supported by a ring 49 preferably of ceramic Yiittedinto sleeveY 34. The tubing 48, supports .the wave helix 44 coaxially with the shield 16 and in align- Y ment with the gun 22 and the electron beam produced thereby.

At the output end of the tube an output coaxial line sec- Mtion or resonator 36 is provided which is similar to the input resonator 36, with a few exceptions. Only the differences in the two resonators will be pointed out, as the similarities will be apparentto those skilled in the art. In resonator 36', parts corresponding to those of resonator 36 bear corresponding numbers, but with a prime. The same travelling wave helix passes through both resonators. ResonatorV 36 has its corresponding parts assembled in reverse order (left to right as viewed in the drawing) from those of resonator 36. One pair of aligned apertures near the Vperiphery of members 38 and 46' permits passage ofthe input line with its outer conductor 14 in immediate contact with shield 16. Another aperture near the periphery of cup member 38' receives an output coaxial transmission'line 10 having an outer conductor 14 and -an inner conductor 12'. The line 10' may be considered the counterpart of the line 10, although it enters the Vresonator through the cup-member 38.

An extension 12a' of inner conductor 12 completes a coupling loop by making contact near the end of the center conductor of the second member 40 of resonator 36. n

No spiral shield, such 'as 32, is required at the tube out- 'i put end. The quartz tubing 48 extends coaxially through .extending across the gap.

In alignment with the helix 44 adjacent the output end, is a collector electrode 54 a lead to which is brought out of the end of envelope at the tube input-output end.

In operatiomsuitable voltages are applied through the pins of base 24 to the electron gun 22. A beam of electronsV is produced -by the gun directed coaxially through the spiral H shield 32 and the helix 44 to the collector 54.' The electro- 'magnet 18 has suitable currents applied to producen magnetie field parallel to the electron beam.

Y This magnetic field aids in keeping the beam diameter small enough to stay VWithin the helix and maintaining Yto the resonator to the coaxial transmission line.

, 4 the parallel flow of electrons of the beam. The metal parts enclosed within the magnet 18 winding are preferably of non-magnetic metal to allow uniformity of the magnetic field along the axis. The input high frequency voltage to be amplified is applied at the input end of the coaxial line 10. The line 10 is coupled to the resonator 36 to transfer energy to and resonate the resonator by the coupling loop formed by the inner conductor extension 12a. i i 'i The folded-back probe 42 couples energy from the resonator 36. The probe 42 is connected for direct current through sleeve 34 or by -direct contact to the spiral shield 32. The spiral shield 32 provides a space eld free with respect to direct-current voltage, but allows penetration of the high frequency elds in the space within the spiral shield. The high frequency iields penetrate to the space within the spiral shield :t2-because the electric vectors are substantially normal to the spirally wound conductor. For low frequency or D. C. voltages, however, the spiral actsas a Faraday cage, and the D. C. elds do not penetrate to any substantial degree. As a result, the electron beam is not distorted as it passes through the spiral shield. All the conductors within the spiral shield 32 including the travelling Wave helix 44 are at the D. C. voltage of the nal accelerating electrode of the electron gun 22. At the same time, the high frequency elds from the resonator 36 penetrate the spiral shield 32 to couple With probe 42 and with the travelling wave helix 44 to Vapply energy at the input end of the helix. The coupling probe V42 is effectively connected for high frequency energy to the open-circuited end 40a of the center conductor of the coaxial resonator by the capacitive coupling of sleeve 34. The sleeve is preferably a quarter wave length at the midband operating frequency for this purpose.

The resonator 36 affordsspecial advantages in making the coupling for high frequency energy between the input line 10 and the travelling wave helix 44. yAs a rule, the helix has a high impedance (for example of 200-'1000 ohms) compared to the impedance of the input line `(for example of 50 ohms) The resonator 36 may be employed asa transformer. The coupling loop 12a from the coaxial line extension is connected to the resonator so that a transformation of the input line impedance through the resonator and coupling probe 42 matches the travelling wave helix 44 impedance. Therefore, the resonator 36 Y acts as a transformer, which is in fact broad band in effect. Through this means the input line 10 in a particular device was matched to the helix with a volta-ge SWR less than 1.5 over a i, 30% frequency band inthe vicinityof 3000 mc./s. The resonator in the form illustrated is readily assembled. It will be observed. that different envelopes 20 complete with enclosed electrodes and helix may be interchangeably inserted in the remainder of the assembly. Thus one magnet, shield, and resonator assembly may be used'to test or operate different tubes. Also, the orientation of insertion with respect to the angular position about the axis of the assembly of. fthe partswithinenvelope 20 is substantially immaterial. The fields in the coaxial resonator 36 are substantially rotationally symmetrical aboutthe tube axis.

The high frequency power applied to helix 44 travels as a waveV toward the helix output end in the same direction as the electron'beam. There is an interaction between ,this wavesand the electron beam, if the velocities are appropriately selected, as is Well known.

At the output'end, the high frequency power is coupled frornthe helix by the coupling probe 42` tothe Voutput resonator 36. The operation of the output resonator 36 is converse of that of the input resonator 36, that is, energy flow is in the direction from the coupling probe It may be noted that the output transmission'line 10 is inserted into the output resonator 36 from an end of the resonator chamber which corresponds to the end' opposite the Yentry of input line 10 intoY the Yinput resonator36. That is, in the one case (the input) the entry is from the end of the second coaxial member 40; in the other case, the entry of the coupled line is from the cup member 3S. Notwithstanding, the coupling loop in each case formed by the extension of the inner conductor of the coupled line is at a point of low impedance to match the resonator to the coupled resonator. On the other hand, the coupling probe 42 is at a point of high concentration of the electric field lines of the high field resulting at the gap 46 or 46 of the center conductor of the resonator, and parallel to said lines at that point. Therefore, the coupling probe is coupled at a point of high impedance in the resonator to match the coupled helix 44 to the resonator.

In the arrangement of Fig. 2, only the input resonator is shown. A similar arrangement for the output resonator of the travelling wave tube is readily understood from the input resonator arrangement illustrated. The resonator 36 may be similar to that of Fig. l, but reversed in axial relation to the electron gun. The coupling probe 42 extends across the gap 46 and is not folded back. The input line 1t) comes into the resonator through aligned apertures in the side of the shield 16 and the side of the resonator 36. The sleeve 34 is within the envelope 20 and the travelling wave helix 44 is supported by the envelope. The arrangement of Fig. l with the probe folded back is preferred because the electron gun 22 may be positioned more closely to the travelling Wave helix 44. Gn the output end, the collector of Fig. l is positioned more closely to the helix than may be accomplished with an arrangement corresponding to Fig. 2 at the output end.

The operation of the device of Fig. 2 should be clear from what has been said hereinbefore. The extension 12a of the inner conductor 12 of line 10 provides a low impedance broad band coupling matched to the line l@ at the operating frequency, as in Fig. l. The extension lZa is tapered to be broader at its base where it connects to the center conductor of resonator 36. This is also true in Fig. 1. Such taper aids in securing broad band results.

The cup member 38 of Fig. 2 may be omitted entirely. The performance with this omission is in the same manner as with the member 38. The end Wall of the resonator 36 remote from coaxial member 40 is physically omitted. At the operating frequencies for which waves are suitably guided Within the travelling wave helix 44, the wave channel which might be supposed to exist between shield 16 and the helix 44 is non-existent. At such frequencies, this supposed Wave channel acts like a Waveguide dimensioned below or less than cut-off dimensions. Physically, a simplified explanation is that at these operating frequencies, the wave velocity in or on the helix is so much less than that of the surrounding shield, that the shield and concentric helix cannot support the propagation of the waves between them. Here the Waves are exponentially decayed, and not propagated. The energy is constrained to travel close to the helix 44, that is, within about three helix radii from the helix axis. Consequently, there is a pseudo-Wall, or virtual wall on the travelling wave helix side of the resonator, at a point corresponding to the removed wall. There are substantial reflections which make a real wall unnecessary. In other respects operation is the same.

ln Fig. 3, the coaxial resonator has its inner tubular conductor 56 connected to probe 12a, but only capacitively connected to the resonator external Walls by coupling to a tubular wall projection 58. The tubular conductor 34 is capacitively coupled at the end of the resonator to the external Wall thereof and extends across the gap between tubular inner conductor 56 and the coaxial tubular projection S8. The extension of member 34 across the gap serves the function of probe 42, by coupling to the resonator elds to which it is exposed.

ln operation, the impedance presented to the coaxial line 10 by the resonator is made to provide a match as near as possible. The helix is also substantially matched by the probe coupling to the resonator. Hence the use of the resonator provides a highly eicient coupling between the coaxial line 10 and the helix 44. The mode of operation will be clear from what has been said heretofore and the structure illustrated in Fig. 3. The virtual wall 38a corresponding to the omission of second member 4t) is indicated by dotted lines.

In Fig. 4, the coupling probe 42 is folded back as in Fig. l. The probe is connected to a capacitive coupling sleeve 34 to the tubular center conductor 56. The spiral shield 32 is omitted from the showing for the sake of clarity of illustration, but may be included if desired. The tubular Wall projection 58 is unnecessary and is therefore omitted.

Fig. 5 illustrates sixteen dierent embodiments of the invention by separately showing (lines 5a and 5j) two different resonators, and eight different coupling probe and helix arrangements (lines 5b to 5h inclusive). Any one of the eight probe and helix arrangements may be used with either one or the other of the resonators. For example, the probe and helix arrangement of Fig. 5b may be positioned with the helix 44 aligned with the coaxial resonator 36. The point A at the end of probe 42 is to be connected to the point A in the resonator 36. The tubular projection 5S is to be removed, as not needed in this combination. Projection 58 of line 59 is also to be omitted in the arrangements resulting from the combination of the resonator 36 of Fig. 5a with the probe and helix arrangements illustrated in lines 5c, 5d, Se, 5f, and 5g.

in all of the combinations illustrated by Fig. 5, the helix is to be located coaxially of the resonator 36. The relative axial positions of the parts is unchanged and is indicated by the two light vertical lines.

ln combining either, lines 5b or 5c, with lines 5a or 5j points A are to be connected. In combining lines 5d and Se with lines 5a or 5j points B are to be connected.

In combining lines 5f and 5g with lines 5a or 5]', points A are to be positioned together, this being the point of the effective capacity coupling of the probe (by means of the sleeve 34 to the tubular center conductor of the coaxial resonator).

in combining lines 5h and S With lines 5a or 5]', points E are to be positioned together. Only in these four arrangements (lines 5h or 5i combined with lines Sa or 5j) is the projection 58 required. In the other cases it is preferred to omit it.

In each of lines 5b to 5i, an appropriate position for the electron gun 22 is illustrated. It will be appreciated that if the coupling is to be employed at the output end of the tube, the electron gun 22 is to be replaced by a suitable collector.

The manner in which a cavity resonator such as that shown in Figs. l, 2, 2a, or line `5a of Fig. 5 and that shown in Figs. 3, 4, or line 5j of Fig. 5 operates to provide the desired transformation of impedances may be explained in la qualitative manner. Referring to line Sa of Fig. 5, the stub 12a is connected at a point an electrical quarter wavelength from the short circuit at the left hand end of the resonator (as viewed in line Sa). This short circuit is imaged as a virtual open circuit 'at the connection of extension 12a. However, to the right hand side there is a shorter section of coaxial line. At the high voltage point across the gap 46 is the probe coupling to the high impedance travelling wave helix. This high im pedance is stepped down or transformed to match the connection of the low impedance coaxial line terminating at the extension i211 by virtue of the intervening length of line `and the gap.

Similarly, line 5j of Fig. 5 makes clear the similarity in operation. The extension 12a is connected to the tubular inner conductor 56 near its left hand end (as quency band.

7 Yviewed-in line j) so that the resonator space Vto the left of the end of that tubular inner conductor appears as an open circuit, both physically and electrically. To the Yright, as in line 5a, the extension 12a sees a short section V5a and 5g. The arrangement of lFig. 2 is the equivalent of the combination lines 5a and 5f. The arrangement of Fig. 2a is the same, Vexcept that the end resonator wall on the helix sideris omitted, which may always be done where a shield suchfas116 is employed. The arrangement is the equivalent of the combination of lines 5j and 5h, but with the end wall on the side of the helix omitfted.` Fig. 3 should be turned right to left to make the equivalence obvious, and the extension of sleeve 34 across :the gap 46, is considered the equivalent of probe 42. The arrangement of Fig'. 4 is the kequivalent of the combination of line 5j with line 5g `and with the end wall on the side of the resonator 36 toward the helix omitted (turn Fig. 4 rightto left to readily see the equivalence). Of the various embodiments, those resulting from a combination of lines 5c and 5g with the resonators of lines 5a and 5] are desir-able because they allow close proximity of the helix and the gun. The combination of line 5g withrlines 5a or Sjis preferred because the capacitive coupling through sleeve 34 permits the direct connection, as through Ythe spiral shield 32, of the last accelerating electrode of the electron gun and the helix,

` tion provides an eicient means of coupling `a coaxial line to a travelling wave tube helix over a wide fre- What is claimed is: l. A coupling arrangement comprising a kresonator having metallic walls which include a tubular outer conductor, a `coaxial tubular innerV conductor having an axial gap and at leastone end wall, a coupling probe extending 'axially across said gap, a helical transmission line axially aligned with Vtherresonator axis and extending into said tubular inner conductor, one end of said helical line being connectedto one end yof said probe, the other end of said probe/being coupled to one of said metallic walls, and a coaxial: lineV having innerand outer conductors, said coaxial line outer conductor being connected to said tubular outer conductor, said coaxial line inner conductor extending in radial ydirection through the space between said resonator tubular conductors and terminating at a connection with said tubular inner conductor. l

2. The arrangement claimed in claim l, said gap being between said end wall and one axial termination of said tubular inner conductor.

3. The arrangement claimed in claim 1, said helical line being the delay line of a traveling wave tube, and said coupling probe being folded back from the said one end of said helical line and positioned axially coextensive with said helical line for the probe length.

4. The arrangement claimed in claim l, said helical line forming the delay line of a traveling wave tube, and said probe being folded back from said one end of said helical line and positioned axially coextensive with said helical line for the probe length, the coupling of the said other end of said probe to said one metallic W-all being a capacitive coupling.

i 5. The arrangement claimed in claim 4, said capacitive coupling 'being Yto the tubular inner conductor.

6. The arrangement claimed in claim 5, `said capacitive coupling to the tubular innerV conductor comprising ya conductive sleeve positioned coaxially `within and spaced from said tubular inner conductor and to which said other probe end is connected.

Y7.V The arrangement claimed in claim l, thecoupling of the4 said other end of said probe to said one metallic wallbeing'a capacitive coupling. f

V8. The arrangement vclaimed in claim 7, said capacitive coupling comprising a tubular sleeve coaxialwith said resonator and to which said other end of said probe is connected. i Y

9. A coupling arrangement as set forth in claim l, wherein the portion of the inner conductor of said coaxial line extending through the space betweensaid reson-ator, tubular conductorsgis tapered, the larger end of said tapered line terminating at said connection with said tubular inner conductor.

10. In a traveling wave tube including a helical delay line of relatively high impedance, a coaxial transmission line of relatively low characteristic impedanceV for supplying Wave energy Vto o'r extracting wave energy from said delay line; and coupling means including la coaxial line cavity resonator resonant at the frequency of said wave energy and having a first portion of relatively low impedance at said frequency and a ysecond portion of relatively high impedance at said frequency, said cavity resonator including a hollow center `conductor portion, said coaxial transmission line being coupled to said portion of relatively low impedance, and one end of said helical delay line extending'into said hollow center lconductor portion of said cavity resonator and being coupled to said portion of relatively high impedance.

1l. In a traveling wave tube including a helical delay line, and electron beam generating means, including a high voltage electrode, for directing an electron beam along the center axis of said helical delay line, a 11ansmission line of relatively lowcharacteristic impedance for supplying wave energy to or extracting wave energy from said delay line; coupling means including coaxial line cavity resonator means resonant at the frequency of said wave energy and having a first portion of relatively low impedance at said frequency and a second portion of relatively high impedance at said frequency, said cavity resonator'being formed with an outer conductorV portion and a hollow center conductor portion, said transmission line being coupled to said rst portion of vsaid resonator, and an'end of'said delay line extending Athrough said hollow center conductor portion and being spaced therefrom, said end of said delay line being coupled to said second portion of -said cavity resonator; and shielding means located in the space between said delay line and said hollowcenter conductor -of said cavity resonator and Yconnectedy to said high voltage electrode,

vsaid shielding means providing shieldingof said delay line with respect to low frequency and direct Vcurrent iields and permitting the passage of elds at the frequency of said Wave energy.

12. In the arrangement set forth in claim l1, said shielding means comprising a helix of wire connected to said high voltage electrode. Y

13. In the` arrangement Vset forth-in claimV 11, said shielding means comprising-a khelix of wire concentrically arranged about said end of Isaid delay line and connected to said high voltage electrode.

14. A traveling wave tube comprising, in combination, a helical delay line of relatively high impedance; electron beam generating means located at oneend of said delay line, including a Vhigh voltage electrode, for directing an electron beam through said delay line; a collector located at the other end of saidY delay lineg-input coaxial transmission line'means of relatively low characteristic impedance for supplying wave energy to `said delay'line; input'coupling means including a coaxial line cavity Vresonator resonant at the frequency of said wave energy formed with a hollow Ycenter conductor portion, .and including a first portion ofrelatively llow impedance at said frequency and a second portionjof Yrelatively high impedance fat :said frequency, said one end of said delay line extending into said hollow center conductor portion of Ysaid cavity resonator and coupled to said second portion of said cavity resonator, the center conductor of said input coaxial transmission line extending radially into said cavity resonator, and said coaxial transmission line being terminated at said rst portion of said cavity resonator; an output coaxial transmission line of relatively low characteristic impedance for extracting wave energy from said delay line; and second coupling means including second coaxial cavity resonator means resonant at said frequency of said wave energy coaxially arranged with respect to said other end of said delay line, said second cavity resonator including a hollow center conductor portion and having a first portion of relatively low impedance at said frequency and a second portion of relatively high impedance at said frequency, said other end of said delay line extending into said hollow center conductor portion of said second cavity resonator and being connected to said second portion of said second cavity resonator, and said output coaxial transmission line being connected to said first portion of said second cavity resonator.

15. In combination, a helix of Wire; a two-conductor transmission line; and means for coupling said helix of Wire to said transmission line including a coaxial line type cavity resonator formed with an outer conductor and a tubular inner conductor, said inner conductor being formed with a coupling aperture, said helix of Wire extending into said tubular inner conductor and being coupled to said cavity resonator at said coupling aperture, one of the conductors of said transmission line being connected to the outer conductor of said cavity resonator, `and the other of the conductors of said transmission line extending in and coupled to said cavity resonator.

16. In the combination as set forth in claim 15, said two-conductor transmission line comprising a coaxial line, the outer conductor of said line being coupled to the outer conductor of said resonator.

17. In the combination as set forth in claim 16, said two-conductor transmission line comprising a coaxial line having an impedance substantially lower than that of said helix of wire, and said coaxial line cavity resonator having an impedance close to that of said coaxial line at the portion thereof to which said coaxial line is coupled, and an impedance close that that of said helix of wire at the portion thereof to which said helix of wire is coupled.

18. An arrangement for coupling an external coaxial transmission line to the conductive helix of a traveling Wave tube, vsaid conductive helix being of relatively high impedance compared to the impedance of said coaxial line, comprising, in combination, a coaxial line type cavity including an outer conductor and a tubular inner conductor, said tubular inner conductor being formed with a coupling aperture, said conductive helix extending into said tubular inner conductor and being coupled to said coaxial line type cavity at said coupling aperture, said coaxial line type cavity acting as an impedance transformer for matching the impedance of said conductive helix to that of said coaxial line, and means coupling said coaxial line to said coaxial line type cavity.

19. A coupling arrangement for a traveling wave tube having a delay line, comprising a coaxial line type cavity formed with an outer conductor and a tubular inner conductor, said inner conductor being formed with a coupling aperture, said cavity being positioned so that said tubular inner conductor surrounds at least a portion of said delay line, said cavity being coupled to said delay line through said coupling aperture, and means for coupling a two-conductor transmission line to said cavity, one of the conductors of said transmission line being connected to said outer conductor of said cavity and the other of the conductors of said transmission line extending in and being coupled to said cavity.

References Cited in the tile of this patent UNlTED STATES PATENTS 2,143,891 Lindenblad Jan. 17, 1939 2,573,460 Lindenblad Oct. 30, 1951 2,580,007 Dohler et al Dec. 25, 1951 2,602,148 Pierce July 1, 1952 2,610,308 Touraton et al. Sept. 9, 1952 2,611,102 Bohlke Sept. 16, 1952 2,637,001 Pierce Apr. 28, 1953 2,694,159 Pierce Nov. 9, 1954 OTHER REFERENCES The Traveling-Wave Tube as Ampliiier at Microwaves, by Kompfner, from Proceedings of I. R. E., February 1947, pages 124-127. (Copy in Div. 51.) 

